Why won't MacBook boot from external drive?

I cloned my internal disk to an external drive so that I can upgrade without recovery disks. Formatting is right, size is fine, disk utility says the new volume is bootable, and it appears as a bootable volume in system preferences. When I reboot and hold option, though, nada. Just the internal disk. Strangely it works perfectly on my girlfriend's 2009 macbook 5,1. Only difference is she has a faster processor and bigger hd. Have tried firewire 400 and usb 2.0 with no luck. Any help is greatly appreciated!

2 Answers

You might try resetting the PRAM and PMU/SMC. Doing these resets has corrected the situation for me in certain cases.

Also -- and this is just a wild guess -- but if your external drive is an exact clone of your internal, the computer may be confusing the two. Just as a test, you might want to pop out your internal drive, and see if it recognizes your external with the internal removed. If this is the case and it does see the external at that point, it wouldn't surprise me if it then started to recognize both after booting from the external once. You could always change the name of the external as well, just so it's not the same as the internal.

It's also possible (but unlikely) that your machine has a firmware password in place. If you are the original owner and you bought the machine new, you can be sure this is not the case. This type of security prohibits booting from an external device in order to prohibit the wiping of the boot HD, and it can be removed by changing the amount of RAM, and immediately resetting the PRAM four times (initial chime, plus three additional chimes). I wouldn't think this is your situation, because a locked computer wouldn't generally allow target or option mode at all (it would show an image of a lock, with a password field below it), but I have seen a few MacBooks out there with this condition.

When you say it worked on the other computer, do you mean the other computer booted from that drive as an external drive? Have you tried putting that drive inside your computer and seeing if it will boot from it internally? Also, Lion will not work on a 1.83GHZ Core Duo, so I'm assuming the OS in question is not Lion....

I'm having the exact same issue, I have a late 2007 white macbook with Lion Compatible hardware. I planned on installing lion to an external hard drive to launch specific software and I didn't want to lose the ability to use snow leopard. The lion installer booted just fine but then when it came to booting up from the external usb drive that I installed lion onto, my macbook wouldn't recognize the disk. Strangely on another core 2 duo machine (iMac late 2009) the hard drive was recognized and I am typing this as we speak on this external drive.